• • An international team of researchers measure the first steps of charge-transfer processes in nitroanilines after photoionization.

• • Researchers reveal the times required for an electron to be transferred from an atom to the adjacent chemical bond and the concomitant structural changes.

• • This study answers a fundamental question in Chemistry: how long an electron takes to initiate charge migration in molecules.
  • The work is a result of the European project TOMATTO, funded by an ERC Synergy grant.

In nature, photosynthesis powers plants and bacteria; within solar panels, photovoltaics transform light into electric energy. These processes are driven by electronic motion and imply charge transfer at the molecular level. The redistribution of electronic density in molecules after they absorb light is an ultrafast phenomenon of great importance involving quantum effects and molecular dynamics. The ability to measure the electron and charge transfer dynamics with extreme temporal resolution not only provides a fundamental understanding of the physical mechanisms behind these processes, but also offers unique insights into how to engineer the chemical and structural properties of the molecule to control or enhance them.

Ultrashort ultraviolet pulses from high-order harmonic sources or free electron laser facilities stand as powerful tools for initiating and observing the response of molecules to photoionization, on timescales ranging from the femtosecond (10^-15 seconds) down to the attosecond (10^-18 seconds). Despite many advancements in these techniques, a detailed understanding of the initial steps of electron and charge transfer after prompt photoionization is not yet available.

In a groundbreaking study published in Nature Chemistry, researchers at Politecnico di Milano, Madrid Institute for Advanced Studies in Nanoscience (IMDEA), Autonomous University of Madrid and Complutense University of Madrid unveil new insights into the ultrafast dynamics of molecular systems using attosecond extreme-ultraviolet pulses. This pioneering work offers a fresh perspective on the complex interplay between electrons and nuclei in donor-acceptor molecules, significantly advancing our understanding of chemical processes at the most fundamental level.

By exposing nitroaniline molecules to attosecond pulses, the research team has been able to observe and analyse the earliest stages of charge transfer with unprecedented precision. This study employs a combination of cutting-edge techniques, including attosecond extreme-ultraviolet-pump/few-femtoseconds infrared-probe spectroscopy and advanced many-body quantum chemistry calculations, to capture the dynamics of these rapid processes.

Precise temporal information on the various steps of the electron and charge transfer process have been thoroughly addressed. Key findings from the research reveal that electron transfer from the electron donor amino group occurs within less than 10 femtoseconds, driven by a synchronized movement of nuclei and electrons. This is followed by a relaxation process that unfolds over a sub-30-femtosecond timescale, as the nuclear wave packet spreads in the excited electronic states of the molecular cation. These discoveries offer valuable new insights into how electron-nuclear coupling influences electron donor-acceptor systems in response to photoionization.

The results reported here answer a fundamental question in chemistry as they unveil the times required to transfer charge from an electron donor unit to the adjacent chemical bond connecting that unit with a benzene ring, and for the concomitant required structural changes that occur. The authors believe that these experimental and theoretical findings pave the way to a better understanding of the textbook diagrams and concepts used to qualitatively predict charge migration in organic molecules.

This study not only sheds light on the intricacies of molecular dynamics but also sets the stage for future research in the field towards advancements in both theoretical understanding and practical applications of attosecond science.

The outcome of a ERC Synergy grant

TomATTO is an ambitious scientific project that aims to capture the ultrafast dynamics of electrons with the aim of improving the conversion efficiency of solar energy. By observing, understanding and controlling the excitation of molecules in solar cells, the researchers say, the performance of these devices could be improved, which currently only manage to convert less than 25% of the solar energy that reaches them.

Over the course of the 6-year project, TomATTO researchers will overcome three major challenges: 1) record the first electronic processes initiated by light absorption; 2) design new organic materials to control electronic dynamics; and 3) develop computational methods to understand the results. To this end, the research team of the TomATTO consortium, led by professors Fernando Martín (IMDEA Nanociencia, Universidad Autónoma de Madrid), Nazario Martín (Universidad Complutense de Madrid) and Mauro Nisoli (Instituto Politécnico de Milán), join forces in this synergistic project. The TomATTO project, coordinated from IMDEA Nanociencia, receives funding from the European Research Council (ERC Synergy) with a total of 12 million €.

This work is the result of a collaboration project among researchers at Politecnico di Milano (Italy), the Institute for Photonics and Nanotechnologies (IFN-CNR, Milan), Madrid Institute for Advanced Studies in Nanoscience (IMDEA Nanociencia, Spain), Universidad Autónoma de Madrid (Spain), IMEC (Leuven), Universidad Complutense de Madrid (Spain), Istituto di Struttura della Materia (Rome), Sincrotrone Trieste (Italy). The work has been partially funded by the ERC Synergy Grant TOMATTO (951224) awarded to professors Mauro Nisoli, Fernando Martín and Nazario Martín; the COST Action ATTOCHEM (CA18222); and the accreditation Excellence Severo Ochoa awarded to IMDEA Nanociencia (CEX2020-001039-S).

Reference

• Federico Vismarra et al. Few-femtosecond electron transfer dynamics in photoionized donor–π–acceptor molecules. Nature Chemistry (2024). DOI: 10.1038/s41557-024-01620-y
• Link to IMDEA Nanociencia repository:  https://hdl.handle.net/20.500.12614/3761